| Summary: | The focus of this thesis is structural analysis of the Billefjorden Fault Zone astride Billefjorden and Sassenfjorden in Spitsbergen, with the use of seismic interpretation combined/aided by geological and bathymetric maps, field observations and well data. The aim is to describe structures and the tectonic development of the study area. The focus is on large tectonic events from Devonian to Palaeogene. The Billefjorden Fault Zone was described in detail by Harland et al. (1974). Since then, many studies have been made on the fault zone, with much focus on along-strike changes and a suggested reactivation history. Most published work is based on land observations from Austfjorden to Pyramiden. Much less work has been done on the offshore domain of the fault zone. Marine seismic data from Billefjorden and Sassenfjorden were available for this thesis. In order to identify stratigraphic units in the seismic profiles, terrestrial seismics are used in combination with well data and a velocity survey from Reindalen. Furthermore, geological maps, published works and bathymetry are used for structural interpretation. The seismic data are of very poor quality. This presents a challenge in locating stratigraphic units and identifying structures. No well-tie is available in the study area. Therefore, a well in Reindalen is used. The distance from the well to the study area is problematic since only part of the stratigraphy overlap. Another problem are significant geological changes along the tie-line. The problems with poor data quality is partially solved by combining the seismic data with geological maps and bathymetry. The result is a number of interpreted seismic lines and bathymetry. The interpretation includes stratigraphic division and structural elements such as faults, folds, horst, graben and basins. Based on the seismic interpretation and bathymetry a suggested tectonic development is presented. The structures described in Billefjorden and Sassenfjorden span form post-Caledonian orogenic collapse, Devonian convergence, Carboniferous basin development, Cretaceous intrusive events and Palaeogene convergence. Furthermore, a model for the offshore continuation of the BFZ in Billefjorden and Sassenfjorden is presented. The model differs from other published work in regard to the southward extent of the Balliolbreen Fault and Odellfjellet Fault. In this thesis it is suggested that the fault array is preserved between Pyramiden and the south coast of Sassenfjorden. An important along-strike change is the narrowing of a fault-bound horst that is bound by the two faults. Analysis of the seismic data and bathymetry also suggest the existence of two NW-SE trending lineaments along Sassenfjorden. The two lineaments are suggested faults which have not been mapped previously.
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